This invention relates to battery chargers, and more particularly, to battery charging systems which are automatically controlled as a function of the sensing of certain battery conditions.
The fast charging of storage batteries involves problems relating to battery temperature. Battery charge acceptance characteristics vary depending primarily upon internal battery temperature. Cold batteries, for example, require a higher applied voltage for a given current level than do warm batteries. A constant charging voltage set to charge a cold battery will overcharge a warm battery thereby releasing thermal energy. This results in the battery temperature increasing, worsening the overcharge condition. Such may result in thermal runaway.
Conversely, a constant charging voltage set to avoid overcharging warm batteries is insufficient to charge cold batteries. The result is an undercharge condition. Repeated undercharge or overcharge, or both, yields shortened battery life.
Ideally, the battery charge voltage should be varied as a function, therefore, of battery temperature. However, overcharge or undercharge may also result from an improper duration of charge time at a given charge voltage. There is, therefore, a proper battery charge time requirement which is variable and is related to the length of time needed to bring a discharged battery up to a given set of recharge conditions.
It is, therefore, an object of this invention to provide a battery charging circuit which controls the level of the constant output voltage being supplied to the battery as a function of the temperature of the battery.
It is a further object of this invention to provide a battery charging circuit which incorporates control means for terminating charging a predetermined time after current being supplied to the battery drops below a predetermined level.
These and other objects of the invention will become apparent from the ensuing description of the preferred embodiment.